We attempted to improve the performance of passivated emitter and rear cells by increasing the quality of the aluminum back surface field (Al-BSF) beneath the local contact. We demonstrate that a thicker Al-BSF can be obtained by adding Si to the screen-printing Al paste, offering a greater penetration depth of several micrometers compared to the silicon-free Al paste, thereby increasing the open-circuit voltage. The presence of Si in the Al paste is believed to suppress the driving force for the strong lateral Si diffusion toward the Al layer during alloying, as evidenced by 50% decrease in the penetration depth of the Al–Si melt. The electron probe microanalyzer element analysis indicates that the Si-free Al paste produced the combination of hypo- and hypereutectic structures, while the use of the Al paste containing Si resulted in a hypoeutectic alloy, which has a strong presence of the primary α -Al dendrites. Different BSF thicknesses can be explained by that the liquid Al containing Si within the openings is saturated faster than the Si-free Al liquid under the same conditions. These findings allow a better understanding of the mass transport phenomena stimulated by the diffusion of Si and Al during the alloying process.